Self-locking dies for making threads

ABSTRACT

This invention relates to self-locking screws and thread rolling dies for making such screws. The self-locking result is achieved by a rib which may be continuous or discontinuous in the flank or flanks of one or more turns of the threads over a selective length of the screw. The outermost face of the rib is generally parallel to the mating threads so that with standard threads it is disposed at approximately 60* with respect to the oppositely disposed flank on the same thread to provide initial compression and subsequent resilient bending of the rib without scoring the mating thread despite variations in fit. The screw threads and ribs therein may be made by the use of conventional flat or circular thread rolling dies in which certain selective threads in at least one of the dies, preferably the movable die, have been reformed over a suitable length in a corresponding manner.

United States Patent [151 11 3,79,644 Qrlomoski Feb. 5, 1974SELF-LOCKING DIES FOR MAKING Primary ExaminerMilton S. Mehr THREADSABSTRACT This invention relates to self-locking screws and threadrolling dies for making such screws. The selflocking result is achievedby a rib which may be continuous or discontinuous in the flank or flanksof one or more turns of the threads over a selective length of thescrew. The outermost face of the rib is generally parallel to the matingthreads so that with standard threads it is disposed at approximately 60with respect to the oppositely disposed flank on the same thread toprovide initial compression and subsequent resilient bending of the ribwithout scoring the mating thread despite variations in fit. The screwthreads and ribs therein may be made by the use of conventional flat orcircular thread rolling dies in which certain selective threads in atleast one of the dies, preferably the movable die, have been reformedover a suitable length in a corresponding manner.

5 Claims, 10 Drawing Figures PATENTED FEB 5 74 saw 2 or 3 Fig-6SELF-LOCKING DIES FOR MAKING THREADS I BACKGROUND OF THE INVENTIONSelf-locking screws are widely used with correspondingly threaded nutsand apertures. The Locking Fastener Section of the Industrial FastenersInstitute has established specifications for the locking characteristicsof such screws. The standards generally contemplate that a self-lockingscrew of a given size when screwed into a nut of corresponding sizewould require no more that a predetermined torque to make the initialinstallation. There is, also, a requirement for a minimum break-awaytorque for the same screw after it has been removed, reinserted andremoved a fixed number of times, and it is desirable that the ratio ofthe initial drive torqueto the final removal torque be relatively low.It should be understood that while the prior art primarily includesself-locking screws, the invention disclosed herein pertains to bothinternal and external locking threads.

Commercially available self-locking threaded fasteners embody in orbetween the threads some kind of yieldable, nonmetallic material thathas been secured to the threaded structure by a secondary operation.Other types utilized deformed ormismatched threads which createincreased friction between the mating threads. A self-locking thread isdescribed in U.S. Pat. No. 3,517,717 issued June 30, 1970. ApplicationSer. No. 26,520 filed Apr. 3, 1970, teaches the dies and the method formaking that thread described in U.S. Pat. No. 3,517,717 and applicationSer. No. 26,529 filed Apr. 3, 1970, discloses still another form ofself-locking thread. Each of these applications and the patent has thesame inventor and assignee as this application. The self-locking threadsdescribed in the patent and application have received wide acceptance inthe fastener market. The present invention 'is an improvement thereoverhaving a generally triangular, cross-sectional rib disposed on the flankof a thread and having a surface thereof cut away topresent a surfacewhich is generally parallel to or aligned with surfaces of the matingthread.

SUMMARY OF THE INVENTION The present invention contemplates a bodyhaving self-locking threads disposed thereon wherein at least one of thethreads has a quadrilateral rib on at least one flank. The rib has abottom wall commencing at the flank intermediate the pitch line and rootof thethread, an inner wall commencing nearer to the intersection of thepitch line and transverse mid-section of said thread than to theconventional line of the flank. The outer wall intersects the inner wallalong an arcuate ridge line which line is generally midway in an axialdirection between the root of the one flank and the crest of said onethread, said outer face of said rib having a surface disposed generallyparallel to the original flank or at an included angle of approximatelysixty degrees with respect to the thread face on the distal side of thethread. In one form the crest of the rib is outside the pitch line ofsaid thread, and the included angle between the faces of the ribadjacent the crest thereof form approximately a right angle beforeassembly to a mating thread. The inner wall may be slightly convex andthe flank upon which the'rib is-disposedmay be slightly concave. In'some forms the inner wall at some place in its width intersects thetransverse plane that defines the mid-section of the thread, and theintersection of the inner wall and the third wall defines a cavityhaving an included angle between the walls of about 77, and a volume notless than one quarter of the volume of the thread including the rib thatis radially outward of the pitch line.

In still another form the member having self-locking threads disposedthereon has at least one of the threads provided with a rib on at leastone flank. The rib has a generally quadrilateral form before assemblywith a meshing thread and is disposed with a base on the flank, innerand bottom walls intersecting the base, and an outer wall joining theinner and bottom walls. The outer wall forms an included angle ofapproximately sixty degrees with respect to the oppositely disposedflank of the thread. Preferably an obtuse angle is formed between thebottom wall and the outer wall. The crest of the rib defined by theintersection of the inner wall and the outer wall is normally outsidethe pitch line of the thread and normally a plurality of ribs will bedisposed on successive threads.

The invention further contemplates a thread rolling die for rolling theself-locking threads of the invention on a cylindrical surface such as ascrew blank. The die has at least one face having a plurality ofgenerally parallel ridges adapted to form corresponding threads on acylindrical workpiece and having at least one interval thereof in whicha portion of the flank of at least one of the ridges is cut and formedsignificantly along a path parallel to the crest away from its normalflank configuration to form a rib extending outwardly beyond said flankwhich has a surface disposed at an angle of approximately with theopposite flank of the ridge and a cavity in the flank coextensive withsaid rib. The rib is disposed adjacent to a cavity in the flank which isequal in volume to the rib and the rib parallels throughout a majorportion of its length, the crest of a die ridge and extends upwardlyfrom the root adjacent to the die ridge.

Accordingly it is an object of the present invention to reduce theinitial drive torque to removal torque ratio between an element havingthe locking thread and a conventionally threaded element.

It is another object to provide a locking thread which does not damagethe mating threads particularly where those threads are manufactured ofrelatively low yield strength material.

It is still another object of the invention to provide a locking threadwhich is inexpensive to manufacture and which may be produced rapidly bymeans of thread rolling apparatus.

It is an object of the invention to provide dies capable of long-livedoperation which will produce locking externalthreads of the inventionand which will function for a large number of rolling operations.

BRIEF DESCRIPTION OF THE DRAWINGS ous rib form;

FIG. 3 is a. longitudinal section to a greatly enlarged scale of athreaded surface showing an illustrative locking rib of the presentinvention disposed on two threads thereof;

FIG. 4 is an axial sectional view to a greatly enlarged scale of thelocking thread engaging a conventionally threaded cooperativelydimensioned internal thread;

FIG. 5 is a transverse sectional broken-away view of a die in alignedrelationship to the corresponding threaded surface in FIG. 3 and takenat line 5-5 of FIG. 7;

FIG. 6 is an end view of a tool for forming the die of FIG. 5 which hasbeen modified with the tool so as to produce a die in accordance withthe invention;

FIG. 7 is a view of the upper movable die from the bottom when the dieis normally positioned for use and showing the location on the surfacesthereof which have been altered by means of the forming tool shown inFIG. 6;

FIG. 8 is a top plan view of a conventional fixed die suitable for usewith the movable die shown in FIG. 7;

FIG. 9 is a partially broken-away isometric view to a greatly enlargedscale of the upper surface of the movable die shown in FIG. 7 andillustrating the position of the rib on the die; and

FIG. 10 is a side elevational view of the forming tool shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2 thereare shown screws 2 and 4 having a multiplicity of conventional threads6. Selected threads 8 on each have been modified from the conventionalform to include a generally quadrilateral rib 14 which may have adiscontinuous or intermittent form as on screw 2 as well as continuousforms as shown on screw 4. Other forms of the invention includeintermittent ribs on two flanks, intermittent ribs staggered from oneflank to the other and continuous ribs on two flanks. The rib may bepositioned on either or both the pressure or nonpressure flanks althoughit is most preferable to provide either a discontinuous or continuousquadrilateral rib on the nonpressure flank of selected threads.

According to the present invention certain of the threads are modifiedin shape in the manner generally indicated in FIG. 3. FIG. 4 shows themeshing relationship of the locking thread with a conventional internalthread. The rib 14 is generally quadrilateral although it should beunderstood that as used herein the sides may be somewhat curved withoutdeparting from the invention. Where a curved face is used a straightline approximating the mean position of the curved line shall beindicative of the angular limitations recited herein. The locking rib 14as shown in FIG. 3 is polygonal in shape, and has an overall triangularform as defined by the dashed line 18 and inner face or wall 26 andbottom face or wall 28. However, the surface of outer wall 24 of the rib14 is cut away to match the mating or cooperating threads so that abearing surface which is generally parallel to the mating thread ispresented. This fourth face 24 provides the quadrilateral shapereferenced hereinabove. This smooth and relatively broad surface 24 hasthe advantage of avoiding scoring or cutting of the cooperating threadsas they are torqued into engagement. The base 18 is coextensive with theflank 20 of an individual thread 8 having an opposite flank 22. Theouter face or wall 24 of the quadrilateral rib 14 is most preferablydisposed at a 60 included angle with respect to flank 22. The verticalcenterline 29 of the thread, sometimes called the transverse midsectionline will, therefore, normally form an angle of 30 with respect to flank22 and outer face 24 of quadrilateral rib 14 in that preferred form.Some variation is possible but this 60 inclination is most preferablebecause conventional threads have that angle between their flanks andstandard physical gaging means utilize the 60 angle as a'reference tocheck other thread dimensions.

A face 24 which is generally parallel to the original flank and to themating threaded element is desirable to avoid stress concentration inthe mating thread although arcuate and other curvilinear forms aresatisfactory as long as the variations from' the parallel form are notso great as to cause stress concentrations. There may be slightcurvature of the outer face 24 of the rib such that the center portionthereof bows away from the flank upon which it is disposed is utilizedin the form of the invention illustrated in FIG. 4. The inner face 26 ofthe rib 14 in the most preferred form of the invention intersects face24 at a right angle.

In the most preferred form the inner face 26 intersects the adjoiningflank 20 of the thread 8 above the pitch line 27 and nearer theintersection with the transverse mid-section line 29 of a conventionalthread than to the line of a conventional thread flank. The anglebetween the inner face 26 and the adjacent flank 20 is most preferablyin the order of 77 and will generally be between 50 and 80 in variousembodiments of the invention. This angle determines the amount ofresilient deflection of the rib which may occur. Inner face 26 and flank20 may be somewhat curvilinear although the angular spacing between thefaces is still important to provide the necessary resiliency. Theintersection of inner face 26 and outer face 24 is ordinarily outsidethe pitch line 27 of the thread 8. In the cross-sectional view of FIG.3, the bottom face 28 of the quadrilateral rib 14 is shown as a straightline although the contour may vary widely without departing from theinvention. Most importantly, the included angle between outer face 24and bottom face 28 is obtuse so that upon deflection of rib 14 nosharply pointed area which would produce stress concentration ispresented to the mating flank 30. In some forms of the invention, theincluded angle may even be degrees. The position of a conventionalthread 31 in FIG. 3 is shown in phantom superimposed on a deformedthread 8 to show the relationship therebetween.

As is apparent from FIGS. 1 and 4, the assembly of threads 8 providedwith ribs 14 with mating conventional threads produces deformation ofrib 14 as it bears against the flank 30 of the mating conventionalthread. The elastic deformation of that rib produces a locking actionwhich prevents undesired loosening of the mating threads. It should benoted that the selflocking feature of the screw as herein disclosed, iseffective with mating threads having varying degrees of tolerances. Itis effective with loose as well as snug fits although it is essentialthat there be interference between the rib 14 and the mating threadwhereby resilient deformation of the rib occurs. Rib 14 extends asufficient distance from flank 20 so that industry torque requirementsare satisfied. g

It should be understood that while the locking thread may be mostinexpensively and rapidly produced by thread rolling on an externalcylindrical surface, the thread may be manufactured by other means oninternal surfaces as well as on external surfaces. Most preferably, themating thread 31 of the nonlocking type is manufactured of a material atleast as hard as the material from which the locking thread 8 ismanufactured. Use of a softer material may result in deformation of thenonlocking thread 31 rather than deflection of the rib 14 on the lockingthread 8. Ordinarily the deformation of the rib in the assembled statewill not be permanent although because the stress varies over thecantilevered rib 14, it is possible that the elastic limit of rib 14 maybe exceeded in certain areas so that the rib form will vary slightlyafter initial assembly. It will be observed in FIG. 4 that the crest 32of the rib 14 is still disposed at a lower point than the crest 34 ofthread 8 despite the deflection of rib 14. The root 70 of the lockingthreads 8 and conventional thread 6 is normally identical.

It is to be noted that although there is some permanent deformationwhich may occur after the initial connection of a locking thread with aconventional mating thread that deformation is not so great that thescrew will lose its effectiveness as a locking threaded connector.Stated in other terms, the resilient nature of the ribs allows repeatedinsertions and removals from the mating thread while retaining thelocking characteristics. The use of the continuous form of ribs 14 asshown in FIG. 2 results in the maximum locking forces for a given numberof altered threads. As will be described hereinafter, the use ofintermittent or discontinuous ribs 14 increases the die life where thethreads are manufactured by thread rolling techniques. Ordinarily, itwill only be necessary to position a rib on approximately six flankscorresponding to the threads in an axial distance equal to six times thepitch. Depending upon the torque requirements, however, a smaller orlarger number'of ribs may be utilized. It is, of course, not necessarythat successive flanks be deformed.

The preferred method of manufacturing the selflocking thread on externalsurfaces is by means of thread rolling because of the great speed andaccuracy which are possible with minimum unit cost. For internal threadsand certain external threads it may be desirable to manufacture theself-locking thread of the invention by means of milling, electricaldischarge machining or other means. Thermoplastics may be readily formedby heating a suitably sized blank and torquing the blank into engagementwith a mating thread. Planetary dies may, also, be utilized tomanufacture the locking thread of the invention although flat dies aresomewhat more preferable because of their reduced cost. Such dies areshown in FIGS. 7 and 8 and are indicated generally by the numerals 52and 54. The construction of these dies and themethod of using them iswell understood in the art and details need not be shown herein. Thegeneral description of the dies, self-locking thread and method formaking each described in commonly owned U.S. Pat. No. 3,517,717 ishereby incorporated by reference. It is sufficient to say that theshorter die 54 is held stationary in a thread rolling machine, thelonger die 52 is moved back and forth parallel to die 54 by the threadrolling machine and a cylindrical workpiece is introduced between theproperly spaced dies, and as it rolls therebetween, the metal of theworkpiece is progressively formed by the teeth of a the dies 52, 54.When the movable dies 52 has carried the rotating workpiece to the endof the short die 54,

the threads of the screw will be fully formed and the completed screwwill fall away.

According to the present invention, it is proposed that certain of thethreads of the die 52 at a suitable location, as for example at the area56, be deformed in the manner generally indicated in FIG. 5. Thetransverse cross-sectional contours of the die 52 taken through thesection 56 will be seen to be complementary to the contours of thelocking thread shown in FIG. 3. The longitudinal positioning of area 56on die 52 has been selected such that when the workpiece has reached apoint between the dies where it is about to roll through area 56, thescrew threads of conventional nature have been almost fully formed andhence are ready to take the deformation provided by the reformed wallsof the threads of the die in area 56. In addition, the longitudinalposition is selected so that the ribs 14 that are formed by die 52 willnot be damaged by die 54. In themost preferred form, the short die 54has similarly reformed portions which assist in forming the rib 14.Where short die 54 has not been reformed, it is desirable to cut away aportion 57 thereof to avoid damage to the deformed thread 8. Thetransverse positioning of area 56 has been selected such that thethreads of the workpiece will be those that are engaged when the lockingthread is coupled to a mating conventionalv thread. Most preferably,they will not be the starting threads so that initial assembly isfacilitated. The number of threads on the die 52 which are reformedwillvary as a function of the torque requirements of the screw as describedhereinbefore.

Turning now to FIGS. 5, 6, and 9 a portion of the die 56 and the formingtool 58 for manufacturing the die are shown. FIG. 5 is a greatlyenlarged cross-section of the movable die reformed area 56 and FIG. 9 isan isometric view of the same area. The forming tool 60 appears in FIG.6. When the thread creating ridges or threads 62, 66 were originallymachined, the flanks thereof are undistorted in the manner of normalthreads. Thereafter, in selected areas, and at selected distances alongthe flanks, the tool 60 of a length to produce the required length ofrib is driven downwardly against the unhardened die flank forcing partof the metal outwardly to create the complement of the entire thread 8including rib 14. This procedure creates a cavity complementary to theforming tool edge 69 as well as surface 88 which is relatively straightalthough not positivelyconstrained by any corresponding surface. Thecomplement includes a major peak 66 and a minor peak 68 correspondingrespectively to root 70 and point 72 defined by the intersection ofinner face 26 and flank 20. Roots 74 of the die 52 correspond to ribpeaks 76. Roots 78 of the die 52 correspond to peak 34 of the lockingthread peak 34. Since this procedure is accomplished before hardening,no heating of the die 52 is necessary. If only one die is reformed, thelength of the forming edge 66 corresponds to the length of the unwrappedlength of the minor peak 68 and root 74 and rib 14 unless that lengthexceeds the unwrapped circumferential length measured at the pitch line.If both dies are reformed, the reformed areas on each may belongitudinally positioned to phase the rib formed by one die to followthe rib formed by the other so that the sum of the length of the ribs onthe two dies is at least as great as the unwrapped length of theexternal thread produced thereby. It will be apprent that the face 82 ofthe die corresponds to face 24 of the locking thread and is disposed atan angle of 60 with respect to flank 84. The minor peak 68 is positionedon a rib defined by faces 86, 88 which intersect at an acute angle.

Movable die 52 and short die 54 are hardened after all of the ribs ofthe selected character and length have been formed. The dies 52, 54 whenused will first create normal screw threads as the workpiece isinitially passed therebetween and then form the locking threads of theinvention with further relative movement.

Various modifications have been conceived to maximize die life. Themodifications to increase die life include increasing the radius of theleading edge of the forming tool which is depressed into the die. Thisaffects the external thread that is rolled with the dies, so that theywill have a correspondingly larger radius formed at rib crest 76. Theuse of a larger radius decreases stress concentration particularly inthe dies which are much more vulnerable to premature failure than thescrew or other externally threaded objects which may be formed with thedies since the'number of cycles ordinarily contemplated for a die exceed100,000 as opposed to a relatively small number of cycles for afastener.

The use of intermittent ribs 14 as shown in FIG. 1 also increases dielife because the rib forming portion tends to increase stress levels.The intermittent ribs 14 are most rapidly manufactured from dies thathave been modified with a forming tool having a plurality ofindentations 80 in the forming edge 69 whereby a discontinuous series ofdepressions are made in the die 52 as shown in FIG. 10.

Still another means for increasing die life is to taper the extremities83 of the forming edge 69 of the forming tool that is used to press intothe die. More specifically as shown in FIG. the relief of this surfacein the forming tool produces an indentation having end extremities ofgradually decreasing depth. This means that externally threaded surfaceswhich are produced with the dies have ribs that are faired into theflanks upon which they are carried.

Various heat treating processes are utilized to maximize die life forparticular applications. The method of manufacturing the self-lockingthreads with the dies will be more apparent by referring to US.application Ser. No. 26,520 filed Apr. 3, 1970, having the same inventorand assignee.

For different sizes of threaded surfaces, it is necessary to vary theangles and position of the rib as will be apparent to those skilled inthe art.

In considering the present invention relative to prior self-lockingthreaded fasteners, reference is again made to applicants US. Pat. No.3,517,717 showing a very thin rib; and to applicants prior developmentof fasteners with a massive or thick rib, as shown in US. Pat.application Ser. No. 26,529 cited above and in an article entitled NewThread Form which appeared at pages 57 and 58 of the Jan. 4, 1971, issueof Product Engineering. Reference is also made to E. M. Ilsemann, US.Pat. No. 2,301,181 granted Nov. 10, 1942. As noted above, certainproblems posed in the self-locking threaded fastener field involve (1)the provision of an inexpensive threaded fastener which will have (2) arelatively low ratio of initial torque to final removal torque afterseveral operations, while (3) accommodating the various mating threadedfasteners of different sizes as permitted by production tolerancestandards for mass produced nuts and screws. It is also desirable tohave a locking thread configuration which will (4) permit use ofrelatively hard self-locking threads with softer mating threads, so thatsteel locking threads could be properly used and reused in aluminum orzinc die castings, for example, without objectionable galling,extruding, or otherwise altering the mating threads.

The E. M. Ilsemann US. Pat. No. 2,301,181 is of interest in showing athread which is provided with protrusions and recesses on the flank ofthe thread for locking purposes. However, only compressive forces comeinto play in the Ilsemann screw, and accordingly the bending ordeflection action forming an important part of the present invention toprovide the high resiliency and resultant low torque ratio is notpresent.

As compared to the prior rib structures as shown in the US. Pat. No.3,517,717, US. Pat. application Ser. No. 26,529, and the ProductEngineering article, the provision of a bearing surface represents afurther advance which is particularly useful in applications involvingsofter metals and in connection with the accommodation of substantialvariations in the size of mating threaded fasteners. It is particularlyto be noted that the present invention contemplates the provision of abearing surface as shown at 24 in FIG. 3 which extends generallyparallel to the original flank of the screw thread so that it makesinitial surface-to-surface area contact with the mating threadedfastener. The use of the bearing surface area distinguishes from theinitial line contact of the prior locking ribs, and produces initialcompressive deformation of the rib without galling or removal of metaleven when relatively softer internal threads are engaged by a harderthreaded screw having the self-locking rib. This permits eithersubstantial compressive deformation of the rib followed by resilientdeflection of the rib in the case of tightly fitting threads, or in thecase of loosely fitting threads, a lesser degree of permanentdeformation resulting from compression but still accompanied or followedby the necessary resilient deflection or bending to achieve the desiredlow ratio of initial to final torque after several on-off cycles.

Those skilled in the art will recognize that the invention provides alocking screw requiring a low initial drive torque while avoiding damageto the mating threads and which is inexpensive to manufacture. Similarlyexperience with the dies will show they are capable of long-livedoperation while accurately producing threads in accordance with theinvention.

1 claim:

1. A thread rolling die for rolling self-locking threads on acylindrical surface, said die having at least one face having aplurality of generally parallel and generally uniformly spaced ridgesadapted to form corresponding threads having a generally uniform pitchon a cylindrical workpiece and having at least one interval thereof inwhich a portion of the flank of one of said ridges is cut and formedsignificantly along a path parallel to the crest thereof away from itsnormal flank configuration to form a rib extending outwardly beyond saidflank having a surface disposed at an angle of approximately sixtydegrees with the opposite flank of said ridge and a cavity in said flankcoextensive with said rib.

2. A thread rolling die as set forth in claim 1 wherein a given lengthof said rib is equal in volume to the adjacent length of said cavity.

3. A thread rolling die for rolling self-locking threads on an externalsurface, said die having a plurality of generally wedge-shaped parallelridges each being spaced from an adjacent ridge a substantially equalamount and said ridges having crests and roots, at least one ridge ofsaid die having at least one straight rib paralleling throughout a majorportion of its length the crest of said one ridge and extending upwardlyfrom the root adjacent said one die ridge and having inner and outerwalls intersecting at an acute angle and disposed 5. The die as setforth in claim 3 wherein the intersection of said rib and said flankadjacent said cavity has a substantial radius therebetween to avoidstress concentration.

1. A thread rolling die for rolling self-locking threads on acylindrical surface, said die having at least one face having aplurality of generally parallel and generally uniformly spaced ridgesadapted to form corresponding threads having a generally uniform pitchon a cylindrical workpiece and having at least one interval thereof inwhich a portion of the flank of one of said ridges is cut and formedsignificantly along a path parallel to the crest thereof away from itsnormal flank configuration to form a rib extending outwardly beyond saidflank having a surface disposed at an angle of approximately sixtydegrees with the opposite flank of said ridge and a cavity in said flankcoextensive with said rib.
 2. A thread rolling die as set forth in claim1 wherein a given length of said rib is equal in volume to the adjacentlength of said cavity.
 3. A thread rolling die for rolling self-lockingthreads on an external surface, said die having a plurality of generallywedge-shaped parallel ridges each being spaced from an adjacent ridge asubstantially equal amount and said ridges having crests and roots, atleast one ridge of said die having at least one straight rib parallelingthroughout a major portion of its length the crest of said one ridge andextending upwardly from the root adjacent said one die ridge and havinginner and outer walls intersecting at an acute angle and disposed toform a rib intermediate roots and crests on a workpiece, said rib on theworkpiece having a surface disposed at an angle of approximately 60*with the opposite flank of said ridge on the workpiece.
 4. The die asset forth in claim 3 wherein said rib is faired back into said ridge atits extreme end portions.
 5. The die as set forth in claim 3 wherein theintersection of said rib and said flank adjacent said cavity has asubstantial radius therebetween to avoid stress concentration.